PURPOSE: Unrestricted kinematically aligned total knee arthroplasty (unKA TKA) strives to restore the pre-arthritic posterior tibial slope (PTS), however consistency of achieving this alignment target is unknown. The present study determined the proportion of subjects with differences in PTS less than 2° from the target and the improvement in patient-reported function after unKA TKA. METHODS: A review of 562 postoperative scanograms identified 99 patients (51 female) with a unKA TKA in one limb, a contralateral healthy limb, and a postoperative axial CT scan. All patients were treated with a primary unKA TKA performed with mechanical instruments where the alignment target was setting the PTS to match that of the medial compartment of the contralateral healthy knee. The PTS of the TKA and the healthy medial tibial plateau were measured and the difference determined. The patient-reported Oxford Knee Score (OKS) measured pre- and post-operative function. RESULTS: The proportion of subjects within a tolerance interval of ±2° of the contralateral healthy knee at 95% confidence was 85%. The median OKS improved from 20 points preoperatively to 47 points (range 18-48) at 15 months postoperatively. Greater differences of the PTS from healthy were unrelated to poorer Oxford Knee Scores. CONCLUSION: Unrestricted KA TKA using manual instruments with caliper verification of resection thickness restored a high percentage of patients within a clinically acceptable tolerance of the posterior tibial slope of the contralateral healthy knee. The median postoperative OKS indicated clinically important improvement in patient-reported function.